Fluid sampling is one useful step used for characterizing a reservoir. In-situ fluid composition analysis can be performed during the fluid sampling, and many properties of interest (e.g., GOR) can be inferred about the formation fluid. Knowledge of these properties is useful in characterizing the reservoir and in making of any engineering and business decisions.
The formation fluid obtained during the fluid sampling has a number of unknown natural constituents, such as water, super critical gas, and liquid hydrocarbons. In addition to these unknown natural constituents, the composition of the formation fluid sample may also include an artificial contaminant (i.e., filtrate including water-based mud or oil-based mud), which has been used during drilling operations. Therefore, during fluid sampling downhole, the fluid initially monitored with a fluid sampling device or other instrument is first assumed to be fully contaminated. Then, the monitored fluid is assumed to go through a continuous cleanup process as more formation fluid is obtained from the area of interest.
During cleanup, repeated density measurements are taken at fixed time intervals, and the density measurements are analyzed to estimate the sample's quality. For example, the repeated density measurements can be used to plot the change in density over time. Characteristics of this density-time plot are then used to assess the contamination level of the fluid being sampled. Once a minimum threshold contamination level is believed to be reached, the sample is then captured and stored in the tool so the sample can be returned to the surface and can undergo additional analysis.
For example, FluidXpert® is software that can analyze density sensor data and can estimate the current level of contamination and the amount of time required to reach a desired level of contamination. Since the filtrate density and the uncontaminated formation fluid density are not known and can only be estimated based on the filtrate properties and the pressure gradient, too much uncertainty is present to make a definitive determination that the desired level of contamination has actually been reached. All the same, even with such uncertainty, the information obtained is considered acceptable for regression trend analysis to estimate contamination.
An example of such an approach is disclosed in U.S. Pat. No. 6,748,328 to Storm, Jr. et al., which discloses a method for determining the composition of a fluid by using measured properties (e.g., density) of the fluid. The quality of a fluid sample obtained downhole is evaluated by monitoring the density of the fluid sample over time. During the sampling process, the density of the sample volume changes until it levels out to what is expected to be the density of the formation fluid. Unfortunately, a point of equilibrium may simply be reached between the amounts of formation fluid and filtrate contamination in the sample volume so that the level of contamination is not really known.
To solve this, Storm, Jr. et al. assumes a mixture for the sampled fluid that has only two components, namely filtrate and formation fluid. In this way, the incremental change in the fluid mixture's density corresponds to an incremental change in the volume fraction of the two fluid components by the difference between the two fluid components' densities. The endpoint values for the mixture's change in density include (1) the density of the filtrate (which can be determined based on surface measurements of the mud system) and (2) the density of the formation fluid (which can be determined from pressure gradient data). In the end, Storm, Jr. et al. can indicate the composition of the mixture (i.e., the relative fraction of filtration in the mixture compared to formation fluid) based on the change in the mixture's density over time.
In addition to monitoring density, pressure, temperature, and the like, various other modules can perform analysis downhole. For example, spectrophotometers, spectrometers, spectrofluorometers, refractive index analyzers, and similar devices have been used to analyze downhole fluids by measuring the fluid's spectral response with appropriate sensors. Although useful and effective, these analysis modules can be very complex and hard to operate in the downhole environment. Additionally, these various analysis modules may not be appropriate for use under all sampling conditions or with certain types of downhole tools used in a borehole to determine characteristics of formation fluid.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.